Some electronics applications, such as the testing of disk drives which have sleep and active modes, PCI express cards with sleep modes, or integrated circuits (ICs), require simultaneous power supply and measurement over a large dynamic range. When measuring current via sense resistors, this would require multiple resistors over a wide range of impedances, or the use of extremely sensitive and costly equipment—for example high sensitivity, low noise, Analog to Digital Converters (ADCs) or Amplifiers for measuring currents, and smoothing/averaging techniques to deal with the low signal to noise ratio at low current states.
Circuits which continuously measure current over a large dynamic range, or for power supply and measurement over a large range, are known. However, the limitations of such circuits are such that continuous measurement over the full range is not possible. Also known are alternative methods for smoothing the transition from one dynamic range to another. For example, U.S. Pat. No. 5,917,331 describes a circuit for power supply and measurement, which incorporates a single resistor in use at one time and a “soft” switching method where the measurement circuit is slowly shorted out and reconnected when changing resistors in order to reduce transient voltages. This allows a large dynamic range of current measurements across the operating sense resistor. However, a limitation of this technique is that it does not continuously measure current, because the soft switch means the measurement circuit is bypassed during range switching. It also means that range switching is a slow operation which can limit its usefulness in some applications. It is also a multiplexed system, which can only use one resistor at a time. U.S. Pat. No. 7,276,893 describes a system for switching dynamic ranges for a measurement circuit, with a set of switchable resistors connected in series to change range, in addition to a ramping circuit in order to reduce transients which are introduced during switching. However, while this system uses variable impedance to allow for different dynamic ranges for the measurement circuit, it also uses a ramping circuit to slowly change the impedance, which introduces the same issue of slow switching as for U.S. Pat. No. 5,917,331.